1. Field of the Invention
The invention disclosed herein relates to the field of thin-film deposition, in general to controlled evaporation of single component elements to create complex multi-element films.
2. Background of the Invention
Looking briefly at the background of the invention, thin-film deposition is typically accomplished by two basic methods: (1) physical vapor deposition (PVD) or (2) chemical vapor deposition (CVD). Although there are several subsets of the above techniques, generically all thin films (micron to submicron) are deposited by one of the two methods. This invention particularly relates to PVD processes and more particularly to the PVD field of evaporation. In PVD based processes, atoms are removed from a source material by some physical technique that adds energy to the system causing atoms to be removed. Examples of PVD techniques include sputtering, resistive evaporation, and electron-beam evaporation. In sputtering, the atoms of the source material are removed by the physical act of colliding argon atoms with the source material. The evaporation technique entails removing atoms from the source material by adding heat until the source material atoms are more stable in a gaseous state than in the liquid or solid state. Sputtering and evaporation are well known PVD processes for which several excellent references are available. (Vossen, Maissel and Glanc).
Generally, sputtering can be characterized as a well-controlled, well-engineered process. Sputtering cathodes, power supplies, and source material targets are available from several vendors. Sputtering has been successfully applied in several thin-film applications including deposition of impermeable films on food packaging, low emissivity (low-e) coatings on residential and commercial plate glass, and decorative coatings. Control of sputtered film uniformity has been engineered into the cathode structure. Negative aspects of sputtering including the high cost of the sputtering systems, that the source (target) utilization is generally poor (20 to 40%), and that there are temporal limitations in creating multi-component films (i.e., more that three elements). More specifically, to sputter multi-component films, individual layers are usually deposited followed by a heat treatment cycle to react the components together, which may require considerable time.
Although generally more difficult to control than sputtering, evaporation, is also used in commercial industrial applications. Evaporation is typically used when the specific film thickness uniformity and composition are not critical. Key advantages of evaporation are the low cost of pellets or wire source materials, the low cost of power supplies and crucibles (as compared to sputtering), and the potential for high source utilization (>50%). However, to achieve a uniform film thickness using evaporation requires multiple evaporation sources. As a result, evaporation is less prevalent for films requiring precise thickness or composition uniformity. However, application of evaporation to complex multi-component films that contain one or more highly reactive species has proved problematic due to the relative consistent rate control and thickness uniformity issues.
As noted above, complex multi-element films are difficult to produce using currently available techniques of sputtering or evaporation. The invention described herein provides a new class of evaporation principles and associated evaporation sources resulting in the creation of uniform, well controlled multi-element thin films.
The explicative example uncovered in this invention is deposition of complex 4 or 5 element direct bandgap semiconductors used for photovoltaics.
Looking briefly at the background for the field of photovoltaics generally relates to the development of multi-layer materials that convert sunlight directly into DC electrical power. In the United States, photovoltaic (PV) devices are popularly known as solar cells—which are typically configured as a cooperating sandwich of p- and n-type semiconductors, wherein the n-type semiconductor material (on one “side” of the sandwich) exhibits an excess of electrons, and the p-type semiconductor material (on the other “side” of the sandwich) exhibits an excess of holes. Such a structure, when appropriately located electrical contacts are included, forms a working PV cell. Sunlight incident on PV cells is absorbed in the p-type semiconductor creating electron/hole pairs. By way of a natural internal electric field created by sandwiching p- and n-type semiconductors, electrons created in the p-type material flow to the n-type material where they are collected, resulting in a DC current flow between the opposite sides of the structure when the same is employed within an appropriate, closed electrical circuit. As a standalone device, conventional solar cells do not have a sufficient voltage required to power most applications. As a result, conventional solar cells are arranged into PV modules by connecting the front of one cell to the back of another, thereby adding the voltages of the individual cells together. Typically a large number of cells, on the order to 36 to 50 are required to be connected in series to achieve a nominal usable voltage of 12 to 18 V.
Although commercial use and interest in thin-film photovoltaics has increased dramatically over the past five years, commercial wide-scale use of thin film PVs for bulk power generation historically has been limited due to PV's low performance and high cost. In recent years, however, performance has been less of a limiting factor as dramatic improvements in PV module efficiency have been achieved with both crystalline silicon and thin-film photovoltaics. The laboratory scale efficiency of crystalline silicon is approaching 20%. Modules ranging from 10 to 14% are currently commercially available from several vendors. Similarly, laboratory scale efficiencies of above 10% have been achieved with thin-film PV devices of copper indium diselenide, cadmium telluride, and amorphous silicon. The efficiency of a thin film copper-indium-gallium-diselenide (CIGS) PV device is now approaching 19%. Additionally, several companies have achieved thin-film large area module efficiencies ranging from 8 to 12%. These recent improvements in efficiency have greatly reduced performance concerns leaving cost as the primary deterrent preventing wide-scale commercial application of PV modules for electricity generation.
Thin-film photovoltaics, namely amorphous silicon, cadmium telluride, and copper-indium-diselenide (CIS), offer reduced cost by employing deposition techniques widely used in the thin-film industry for protective, decorative, and functional coatings. Common examples of low cost commercial thin-film products include water permeable coatings on polymer-based food packaging, decorative coatings on architectural glass, low emissivity thermal control coatings on residential and commercial glass, and scratch and anti-reflective coatings on eyewear.
Of all thin film PV compositions, CIGS has demonstrated the greatest potential for high performance and low cost. More specifically, CIGS has achieved the highest laboratory efficiency (18.8% by NREL), is stable, has low toxicity, and is truly thin-film (requiring less than two microns layer thickness). These characteristics allow for the large-scale low cost manufacturing of CIGS PVs thereby enabling the penetration by thin-film PVs into bulk power generation markets.